I've learned a lesson from Wikipedia - people need a stable form of a document to cite for research. I don't imagine too many people cite this essay, but just in case, I've decided to start archiving older versions, that will not be changed. The last entry in the list below will always be a mirror of the latest version of the essay. If you want to cite the essay, use the url of that mirror, and the information will be same as long as I maintain this website.